It is understood that all engines transmit vibrations to the frame of the vehicle. However, the degree of vibration which is transmitted depends upon the manner in which the engine is mounted to the frame.
An engine has the natural tendency during operation to rotate about an axis which has the least moment of inertia. This axis is known as the "least energy" axis. The least energy axis is illustrated in FIG. 1 of the application, and generally extends in the longitudinal direction of the engine, passing through the center of gravity of the engine and rising towards the front of the engine at about a 12.degree. angle. In mounting an engine, there are two competing concerns. The first is the concern that the engine be securely mounted to the frame in a stable manner, while the second is that the engine be mounted to the frame in such a manner as to permit the engine to vibrate about the least energy axis. The latter concern is important because it is commonly recognized that an engine will transmit the lowest level of vibration to the vehicle frame if it is elastically mounted so that it can freely pivot, and therefore vibrate, about the least energy axis. Failure to permit such free vibration results in transmission of various vibration modes to the frame.
Truck engines are generally mounted so that the rear portion of the engine is mounted in two transversely spaced locations, on either side of the flywheel housing. At this longitudinal position, the least energy axis is substantially between the two mounting points, such that the virtual mounting point approximately coincides with the least energy axis. Therefore, the engine is free to rotate about the least energy axis such that the transmission of vibration to the vehicle frame is minimized.
The point at which the front of the engine is generally mounted is below the least energy axis at the longitudinal position of the mounting point. Therefore, when the engine tends to vibrate, or pivot, about the least energy axis the mounting point of the engine tends to move laterally by an arc length corresponding to the angle that the engine pivots. If the front engine mount resists such lateral movement, the engine vibrates in modes other than simple rotation and these vibrations are transmitted to the vehicle frame.
To correct this problem, it is common to utilize a dual mounting arrangement similar to the rear mounting arrangement. In particular, it is known to mount the front of the engine in two transversely spaced locations where the virtual, or equivalent, mounting point coincides with the least energy axis at that position. Additionally, efforts have been made to mount the front of the engine using a single mounting point which has lateral compliance. For instance, U.S. Pat. No. 2,044,714 discloses a front-end mounting arrangement where a pair of rubber blocks are disposed between an engine bracket and seat in such a manner as to permit the engine to pivot about point. Such an arrangement restrains the pivotal movement of the engine about the pivot axis. Thus, vibrational forces are exerted from the motor 1 on the frame 3 through the rubber blocks 7 in a transverse direction.